Two principal capacitive sensing and measurement technologies are currently employed in most touchpad and touchscreen devices. The first such technology is that of self-capacitance. Many devices manufactured by SYNAPTICS™ employ self-capacitance measurement techniques, as do integrated circuit (IC) devices such as the CYPRESS PSOC™ Self-capacitance involves measuring the self-capacitance of a series of electrode pads using techniques such as those described in U.S. Pat. No. 5,543,588 to Bisset et al. entitled “Touch Pad Driven Handheld Computing Device” dated Aug. 6, 1996.
Self-capacitance may be measured through the detection of the amount of charge accumulated on an object held at a given voltage (Q=CV). Self-capacitance is typically measured by applying a known voltage to an electrode, and then using a circuit to measure how much charge flows to that same electrode. When external objects are brought close to the electrode, additional charge is attracted to the electrode. As a result, the self-capacitance of the electrode increases. Many touch sensors are configured such that the grounded object is a finger grounded through the human body, where the body is essentially a capacitor to a surface where the electric field vanishes, and typically has a capacitance of around 100 pF.
Electrodes in self-capacitance touchpads are typically arranged in rows and columns. By scanning first rows and then columns the locations of individual capacitance changes induced by the presence of a finger, for example, can be determined. To effect accurate multi-touch measurements in a touchpad, however, it may be required that several finger touches be measured simultaneously. In such a case, row and column techniques for self-capacitance measurement can lead to inconclusive results.
One way in which the number of electrodes can be reduced in a self-capacitance system is by interleaving the electrodes in a saw-tooth pattern. Such interleaving creates a larger region where a finger is sensed by a limited number of adjacent electrodes allowing better interpolation, and therefore fewer electrodes. Such patterns can be particularly effective in one dimensional sensors, such as those employed in IPOD click-wheels. See, for example, U.S. Pat. No. 6,879,930 to Sinclair et al. entitled Capacitance touch slider dated Apr. 12, 2005.
The second primary capacitive sensing and measurement technology employed in touchpad and touchscreen devices is that of mutual capacitance, where measurements are performed using a crossed grid of electrodes. See, for example, U.S. Pat. No. 5,861,875 to Gerpheide entitled “Methods and Apparatus for Data Input” dated Jan. 19, 1999. Mutual capacitance technology is employed in touchpad devices manufactured by CIRQUE™. In mutual capacitance measurement, capacitance is measured between two conductors, as opposed to a self-capacitance measurement in which the capacitance of a single conductor is measured, and which may be affected by other objects in proximity thereto.
Low-dissipated-power integrated capacitive touch screen controllers with high system noise immunity are highly desirable in many capacitive touch screen applications. Low power dissipation and high system noise immunity typically are mutually contradictory objectives in many known capacitive touch screen controllers, however. The most successful capacitive touch screen controllers generally employ high voltage drive signals obtained from low voltage power supplies by implementing an integrated charge pump that exploits large value electric capacitors external to the integrated circuit. Most of the system noise in such controllers is in fact Electro-Magnetic Interference (or “EMI”) picked up from the touch screen itself.
What is needed is a capacitive touchscreen system that employs relatively low drive voltages but retains high noise immunity.